This question appears to be off-topic. The users who voted to close gave this specific reason:

"Questions asking us to recommend or find a tool, library or favorite off-site resource are off-topic for Stack Overflow as they tend to attract opinionated answers and spam. Instead, describe the problem and what has been done so far to solve it." – legoscia, hexacyanide, Ilya, toniedzwiedz

May I add that they seem incredibly reluctant to fix problems in areas that they're not focusing in. I use Slim templates, and there're these ongoing problems ever since the last major version (7.0) was introduced. It's been around a year now, and they're nowhere near to tackle them. Rubymine is awesome, truly, but without full Slim support it makes it totally useless to me.
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user3209270Jul 20 at 4:01

NOTE: As of NetBeans IDE 7.0, support for Ruby and Ruby on Rails is no longer available in the standard NetBeans IDE build. Please see the Ruby Support Wiki page for more information.
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tivoOct 3 '11 at 20:11

Can Sublime Text 2 be categorized as an IDE?
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Marius ButucOct 30 '12 at 23:26

If you consider all the plugins you can get with Package Control, I would say yes. This is really all I use when working with rails. Sublime text 2 and the terminal. It is actually the best solution I have found.
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Cort3zOct 31 '12 at 7:30

While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes.
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JonAug 29 '12 at 23:41

Surprisingly I like using a few Windows Command Prompts, Explorer, and many Scite instances (with a custom configuration). Not really an IDE but I've found it more flexible and useful than any other IDE that I've tried.

I like Komodo for PHP and Python development but its support for Ruby is pretty thin. I've only recently started working with Rails and Komodo has been disappointing. I'm giving Aptana a try based on the accepted answer.